The discovery of this cue to facial age perception may partly explain why cosmetics are worn the way they are, and it lends more evidence to the idea that makeup use reflects our biological as well as our cultural heritage, according to Russell.

In one study, Russell and his team measured images of 289 faces ages 20-70 years old, and found that through the aging process, the color of the lips, eyes and eyebrows change, while the skin becomes darker. This results in less contrast between the features and the surrounding skin – leaving older faces to have less contrast than younger faces.

The difference in redness between the lips and the surrounding skin decreases, as does the luminosity difference between the eyebrow and the forehead, as the face ages. Although not consciously aware of this sign of aging, the mind uses it as a cue for perceiving a person's age.

In another study involving more than a hundred subjects in Gettysburg and Paris, the scientists artificially increased these facial contrasts and found that the faces were perceived as younger. When they artificially decreased the facial contrasts, the faces were perceived as older.

Cosmetics are commonly used to increase aspects of facial contrast, such as the redness of lips. Scientists propose that this can partly explain why makeup is worn the way that it is – shades of lipstick that increase the redness of the lips are making the face appear younger, which is related to healthiness and beauty.